Acid mine drainage AMD is the outflow of acidic water from metal or coal mines. It occurs when sulfide minerals, such as pyrite FeS2 , are exposed to water and oxygen, resulting in the formation of sulfuric acid H2SO4 . This acidic water can then dissolve other minerals and metals, leading to the contamination of nearby water sources and a decrease in the pH level of the water.The pH level of water is a measure of its acidity or alkalinity, with a pH of 7 being neutral. Acidic water has a pH less than 7, while alkaline water has a pH greater than 7. When water is impacted by AMD, its pH level can drop significantly, sometimes to as low as 2 or 3, making it highly acidic.When acidic water comes into contact with minerals and rocks in the environment, various chemical reactions can occur. Some of these reactions include:1. Dissolution of minerals: Acidic water can dissolve minerals, such as carbonates e.g., calcite, CaCO3 and hydroxides e.g., goethite, FeO OH . This can lead to the release of metal ions, such as calcium Ca2+ , iron Fe2+/Fe3+ , and aluminum Al3+ , into the water. For example, the reaction of calcite with sulfuric acid can be represented as:CaCO3 s + H2SO4 aq CaSO4 aq + CO2 g + H2O l 2. Oxidation of metal sulfides: Acidic water can oxidize metal sulfides, such as pyrite, to form metal ions and sulfate ions SO42- . This reaction can further lower the pH of the water and release more metal ions into the environment. The oxidation of pyrite can be represented as:FeS2 s + 7/2 O2 g + H2O l Fe2+ aq + 2 SO42- aq + 2 H+ aq 3. Precipitation of metal hydroxides: As the pH of the water increases e.g., due to dilution or neutralization , metal ions can react with hydroxide ions OH- to form insoluble metal hydroxides, which can then precipitate out of the water. For example, the precipitation of iron III hydroxide can be represented as:Fe3+ aq + 3 OH- aq Fe OH 3 s These chemical reactions can have significant environmental impacts, such as the contamination of water sources, the destruction of aquatic habitats, and the formation of unsightly deposits on rocks and streambeds. To mitigate the effects of AMD, various treatment methods can be employed, such as the addition of alkaline materials e.g., limestone to neutralize the acidity, the use of constructed wetlands to promote the precipitation of metals, and the installation of barriers to prevent the infiltration of acidic water into the environment.